Polymer electrolytes are useful in electrochemical cells because such solid or semi-solid electrolytes can eliminate the need for microporous separators, and the resultant cell is free of liquid electrolyte. Microporous separators are used in conventional liquid organic electrolyte cells, and are an expensive component of the cell. Also, these separators can increase the internal resistance of the cell and consequently, may decrease the high rate performance of the cell.
The liquid organic electrolytes used in conventional cells are often highly volatile liquids. If these liquids escape from the cell, a hazardous situation can occur.
Typical polymer electrolytes are comprised of a polymer matrix material and an ionically conductive salt. Examples of polymer matrix materials include the polyalkylene oxides, such as polyethylene oxide, polypropylene oxide, substituted polyethylene glycols, and the like. Examples of the ionically conductive salts include LiCF.sub.3 SO.sub.3, and LiClO.sub.4.
There are several examples of solid polymeric electrolyte cells. U.S. Pat. No. 4,925,751, issued May 15, 1990 to Shackle et al. discloses a solid state laminar cell comprised of an alkali metal anode, a solid ionically conducting electrolyte, a composite cathode and a carrier. The composite cathode is comprised of active material, a conductive filler, and an ionically conductive electrolyte which is comprised of a liquid monomeric or prepolymeric curable composition and an ionizable alkali metal salt. The cathode mixture is applied to a current collector and then a layer of polymer electrolyte is coated over this mixture. The polymer electrolyte layer and the polymer in the cathode are cured in place and a thin foil anode layer is then applied on the polymeric surface. U.S. Pat. Nos. 4,748,542, issued May 31, 1988 and 4,638,407, issued Jan. 20, 1987, both to Lundsgaard, disclose solid state capacitors and multicell batteries in which a layer of the solid polymer electrolyte is laminated to a layer of the cathode and anode.
Another example of solid electrolyte cells is disclosed in U.S. Pat. No. 4,594,299, issued Jun. 10, 1986 to Cook et al. The cell is made by protecting an electrode, such as lithium, with an ionically conductive polymer. This electrode can then be combined with a cathode to form the electrochemical cell.
Also, U.S. Pat. No. 4,654,279, issued Mar. 31, 1987 to Bauer et al. discloses a solid polymeric electrolyte which is a two phase interpenetrating network of a mechanically supporting phase of a continuous network of a crosslinked polymer and an ionically conducting phase. A suitable amount of a metal salt is added to the electrolyte to provide ionic conductivity.
In U.S. Pat. No. 4,589,197, issued May 20, 1986, to North, a composite cathode sheet is formed from a solution of active cathode material and a low molecular weight polymer, such as polyethylene oxide, in a solvent such as acetonitrile. The polymer preferably contains an electrolyte salt.
Despite their advantages, there are certain problems associated with the use of solid polymer electrolytes. Three common problems are low volume of active cathode material, mechanical stability of the electrolyte and safety problems associated with assembling live high area cells without the ability to check for shorts. For example, the cells described in U.S. Pat. Nos. 4,925,751, 4,748,542 and 4,638,407 will exhibit each of these problems. The cathode mixture is comprised of the prepolymer and the ionizable salt. Therefore, this mixture will not have as high a volume of active cathode material. The polymer electrolyte layer also contains the ionizable salt, and when it is applied as a film to the cathode mixture, it will be in the form of a tacky viscous gel. Thus, it will be difficult to work with and susceptible to puncture by any of the other cell components, such as the metallic current collector. It is also ionically conductive, and so, when the cathode and electrolyte layer combination is assembled with the anode, the assembly is an active electrochemical cell. In U.S. Pat. No. 4,589,197, the choice of polymers is practically limited to low molecular weight polymers since the polymer is dissolved in a solvent and cast to form a sheet.
Considering these problems, new polymer electrolyte cells and processes for making them are desired.